Owner Neil Edwards had backed the vessel away from the dock without incident, Thomson said. But when he shifted into forward, it exploded.

The passengers all were blown off the boat or jumped, Thomson said. Neighbors helped them out of the water.

Passengers Larry McPherson, 57, and Dawn Van Deursen, 49, both of Escondido, Calif., were flown by State Police helicopter to the Johns Hopkins Bayview Burn Center in Baltimore for treatment of burns to their arms and legs, Thomson said.

They had been in the stern of the boat, nearest the engine, Thomson said. Their injuries were not expected to be life-threatening.

Boat owner Neil Edwards, 54, and his wife, Bonnie Edwards, 48, were taken to Peninsula Regional Medical Center in Salisbury, Thomson said. The Edwards live in Springfield, Va., but have a vacation home in Ocean Pines, 10 miles northwest of Ocean City.

Shannon Edwards, 18, was treated at the scene, Thomson said. She had been at the bow of the boat, farthest from the engine.

The Ocean Pines Fire Department put out the flames and hazardous materials workers cleaned up debris and oil, Thomson said. The boat was destroyed.

Investigators do not know why the boat exploded, Thomson said. Neil Edwards had opened hatches and turned on a blower to disperse any fumes 10 or 15 minutes before he started the engine, she said.

Natural Resources Police and the Worcester County Fire Marshal are investigating.